Fix issue with Flask instrumentation when a request spawn children threads and copies the request context

CHANGELOG.md

@@ -29,6 +29,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Fix Flask instrumentation to only close the span if it was created by the same thread. `ValueError: generator already executing`
+  ([#1654](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1654))
 - Fix TortoiseORM instrumentation `AttributeError: type object 'Config' has no attribute 'title'`
   ([#1575](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1575))
 - Fix SQLAlchemy uninstrumentation

instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/__init__.py

@@ -238,8 +238,8 @@ def response_hook(span: Span, status: str, response_headers: List):
 API
 ---
 """
-
 from logging import getLogger
+from threading import get_ident
 from time import time_ns
 from timeit import default_timer
 from typing import Collection
@@ -265,6 +265,7 @@ def response_hook(span: Span, status: str, response_headers: List):
 _ENVIRON_STARTTIME_KEY = "opentelemetry-flask.starttime_key"
 _ENVIRON_SPAN_KEY = "opentelemetry-flask.span_key"
 _ENVIRON_ACTIVATION_KEY = "opentelemetry-flask.activation_key"
+_ENVIRON_THREAD_ID_KEY = "opentelemetry-flask.thread_id_key"
 _ENVIRON_TOKEN = "opentelemetry-flask.token"
 
 _excluded_urls_from_env = get_excluded_urls("FLASK")
@@ -398,6 +399,7 @@ def _before_request():
         activation = trace.use_span(span, end_on_exit=True)
         activation.__enter__()  # pylint: disable=E1101
         flask_request_environ[_ENVIRON_ACTIVATION_KEY] = activation
+        flask_request_environ[_ENVIRON_THREAD_ID_KEY] = get_ident()
         flask_request_environ[_ENVIRON_SPAN_KEY] = span
         flask_request_environ[_ENVIRON_TOKEN] = token
 
@@ -437,10 +439,17 @@ def _teardown_request(exc):
             return
 
         activation = flask.request.environ.get(_ENVIRON_ACTIVATION_KEY)
-        if not activation:
+        thread_id = flask.request.environ.get(_ENVIRON_THREAD_ID_KEY)
+        if not activation or thread_id != get_ident():
             # This request didn't start a span, maybe because it was created in
             # a way that doesn't run `before_request`, like when it is created
             # with `app.test_request_context`.
+            #
+            # Similarly, check the thread_id against the current thread to ensure
+            # tear down only happens on the original thread. This situation can
+            # arise if the original thread handling the request spawn children
+            # threads and then uses something like copy_current_request_context
+            # to copy the request context.
             return
         if exc is None:
             activation.__exit__(None, None, None)